This invention relates to a floating caisson for use in offshore well operations including drilling, production and oil storage in deep water locations such as seven hundred feet or more.
Prior proposed offshore apparatus for well operations have included vertically moored tension leg platforms in which anchor lines are parallel or substantially parallel and vertically arranged. Such anchor lines are under high pretension to prevent the lines from going slack when waves pass through the platform structure. Failure of an anchor line may not only jeopardize the integrity of the platform, but also the risers connected therewith. The vertically moored tension leg platform is not adapted for laterally controlling the position of the platform relative to a sea floor template by adjusting tension or by adjusting the length of the anchor lines. Such a vertically moored tension leg platform is not suitable for connecting a riser to a sea floor well head by laterally positioning the vessel on the surface by use of anchor lines. Examples of tension leg platforms in the prior art are U.S. Pat. Nos. 3,648,638 and 3,780,685.
Another prior proposed offshore apparatus for well operations includes a floating vessel or semi-submersible vessel equipped with conventional catenary mooring lines which extend from the vessel to anchors on the sea floor which are often a substantial horizontal distance from the vessel. A usual conventional catenary mooring line may have a scope of at least 3:1, that is, a horizontal distance of 3 to a vertical distance of 1. In some instances the scope may be as much as 7:1. An anchor pattern for such a floating or semisubmersible vessel will cover a very wide sea floor area. Such an anchor pattern may cause problems in sea floor installations because of the fouling of the anchor lines with other subsea well equipment. Further, in deep water operations with such a conventional catenary moored floating vessel, a small watch circle, that is the sea floor area designated by the arrangement of anchor means is not possible or feasible. Examples of such conventional catenary mooring lines are shown in U.S. Pat. Nos. 3,778,854 and 3,360,810.
In such offshore operations the platforms are provided with a connection to a riser system which extends from the platform or floating vessel to the sea floor for connection to a well head or other subsea well installation. Such riser systems require tensioning means comprising sheaves, wire rope and hydraulic cylinders to maintain a relatively constant tension on the end of the wire rope to provide the necessary upward vertical force to support the riser means. Such prior proposed riser tensioning means are mechanical devices which are subject to wear and require continuous maintenance. They also occupy substantial space under conditions where space is usually limited by the design of the platform or vessel. In some riser tensioning systems flotation devices are employed and are attached adjacent to the upper end of the riser. In such flotation tensioning systems the riser system is generally exposed to wave forces which in acting on the flotation unit result in undesirable stresses in the riser. An elongated well head structure which is buoyant and which receives therewithin a riser supported by a flotation means is disclosed in Daniell U.S. Pat. No. 3,470,838.
In some instances the riser tensioning means includes a combination of flotation units and tension means in which floats are attached along the length of the riser to partially support the riser weight; and the remaining riser weight is supported by hydraulic tensioning means at the platform as mentioned above.
Storage of oil at sea has included spar buoy type constructions such as shown in Rusking U.S. Pat. No. 3,360,810 and Kapteijn, et al. U.S. Pat. No. 3,921,557. Spar buoy constructions have also been used for mooring and oil transfer purposes in water depths much less than deep water (seven hundred feet or more).